Global Path in Helmet Content Security Policy is not functioning as expected

I recently implemented Helmet to establish content security policies for my web application using Express in the backend. The specific policies are as follows:

const express = require("express");
const app = express();
const helmet = require('helmet');

app.use(helmet());
app.use(
  helmet.contentSecurityPolicy({
    directives: {
      defaultSrc: ["'self'"],
      scriptSrc: ["'self'", "https://ajax.googleapis.com"],
      imgSrc: ["https://firebasestorage.googleapis.com"],
      objectSrc: ["'none'"],
      styleSrc: ["'self'", "https://maxcdn.bootstrapcdn.com/bootstrap", "https://www.w3schools.com"],
      upgradeInsecureRequests: [],
    },
  })
);

However, I encountered an issue when my app attempted to access a link like 

https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/css/bootstrap.min.css
. It triggered a violation of the styleSrc policy. Even though I had specified that 
https://maxcdn.bootstrapcdn.com/bootstrap
should be allowed, I assumed that its child source 
https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/css/bootstrap.min.css
would also be permitted. Surprisingly, it was blocked. How can I enable the child source to pass through then? I attempted using 
https://maxcdn.bootstrapcdn.com/bootstrap*
, but it proved to be invalid.

javascriptexpresscontent-security-policywebsecurityhelmet.js

Answer №1

Greetings from the creator of Helmet!

If you're facing issues, consider adding a trailing slash to your URL, like so:

https://maxcdn.bootstrapcdn.com/bootstrap/

This simple adjustment can make a big difference. For example, using /bootstrap may not allow you to access resources like 

/bootstrap/3.4.0/css/bootstrap.min.css
, whereas /bootstrap/ will. Remember, this is more related to Content Security Policy rather than Helmet itself.

For in-depth information, refer to step 11 of the CSP spec's "Matching Source Expressions" section:

When dealing with source expressions containing a path, and the URL is not a result of a redirect, follow these steps:

  1. Check if the path doesn't end with a slash character.
  2. Split the path into individual items.
  3. Compare the path to the URL path, ensuring they match up to a certain extent.

On a side note, consider streamlining your Helmet code as shown below:

app.use(helmet({
  contentSecurityPolicy: {
    directives: {
      // ...
    },
  },
}));

Instead of using both helmet() and helmet.contentSecurityPolicy(), it's best practice to stick with just one for consistency.

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